This chapter determines variables related to mortality of Legionnaire's disease (LD). An observational, comparative study was performed from 1983 to 2005, with 408 cases of Legionella pneumonia being diagnosed during this time. The variables studied included demographic characteristics, individual and aspiration risk factors, clinical manifestations, radiological manifestations, diagnostic data, treatment, and outcome. The variables were analyzed using univariate and multivariate logistic regression analysis by SPSS version 12.0. Regarding demographic data and risk factors, patients who died had a higher frequency of underlying diseases than the control group (89.8 versus 66.9%): mainly chronic respiratory disease (42.9 versus 29.6%), chronic heart failure (22.4 versus 10.2%), neoplasm (38.8 versus 15.8%), neurological sequelae (20 versus 7.6%), and inmunosuppressive therapy-mainly corticoids (45.8 versus 14.6%). Despite the improvement in the diagnostic procedures of LD and the use of more efficacious antibiotics against Legionella, chronic heart failure, hematological cancer, and corticotherapy are still bad prognostic factors of LD. Recommendations for prevention of LD should focus on settings in which there are persons at greatest risk for illness or serious outcome.